The invention relates to a device for measuring variations of position between two positioning reference systems located at different heights.
Various types of reference systems, i.e. devices acting as reference for positioning elements in space, are known in the field of topographic measurement. Certain reference systems are three-dimensional, others are one- or two-dimensional.
A conventional standard three-dimensional reference system in the field of topographic measurement is formed by a metrological sphere of calibrated diameter, known under the name of Taylor or Hobson sphere, acting as an optic sight.
When we are only interested in levelling of elements in space, i.e. when all that is required is to measure or monitor the relative altitude of each element with respect to the reference system, the latter can advantageously implement hydrostatic means. An example of such a device is disclosed in the document FR 2,656,418. The device comprises closed pots placed respectively at each of the points to be measured. These pots are partly filled with a liquid and are connected by liquid conducting tubes opening out at the bottom so as to form a liquid circuit, and by gas conducting tubes. Each pot is equipped with a capacitive proximity sensor placed above and at a distance from the upper surface of the liquid it contains and designed to supply a position signal in the vertical direction of an area of this upper surface of the liquid in this pot. The device comprises in addition means for comparing the signals supplied by the sensors so as to be able to provide information on the relative position of the pots, and consequently of the points, with respect to one another or with respect to a vertical reference position. Each of the pots of such a device constitutes a levelling reference system in so far as the altitude (absolute measurement) or difference in level (relative measurement) of its base with respect to that of a reference pot is accessible.
The foregoing devices do not however cover all the requirements of precise measurement and monitoring of the level differences or level difference variations between points situated at large distances from one another. Certain large civil engineering structures do in fact require levelling reference systems to be set up situated at different heights. Such is the case for example in dams, mine shafts or underground particle accelerators. Two levelling reference systems situated at different altitudes, at a distance of several meters, then have to be able to be joined to one another to be able to establish the relative variations of their co-ordinates. This connection must be made without adverse implications on existing levelling reference systems and therefore has to be able to be adapted to the latter.
The object of the present invention is to provide a solution to this problem by means of a device for measuring the variation of position of a lower positioning reference system with respect to an upper positioning reference system located above the former, and comprising:
a pendulum comprising
a pendulous weight and
a metrological wire fixed on one side to the pendulous weight and on the other side to the upper reference system, and
a first proximity sensor designed to supply a signal representative of the position or the variation of position in the vertical direction of a flat area of the surface of the pendulous weight with respect to the lower reference system.
In this form, the invention finds a direct application in measurement of mining subsidence. The positioning reference systems may be of any type. Naturally the invention is used to full advantage when the reference systems are at least levelling reference systems, i.e. when they can act as reference for relative altimetric measurements. In practice, the flat area of the surface is preferably rectified and arranged perpendicularly to the wire.
The device preferably comprises in addition a second proximity sensor designed to supply a signal representative of the position or the variation of the position of an area of the wire with respect to a first flat vertical measuring surface securedly affixed to the lower reference system. The device then enables the horizontal movements of the lower reference system in the axis perpendicular to the vertical measuring plane to be monitored. Typically, such a device can be used to perform monitoring of the thrust of the arch of a dam.
According to one embodiment of the invention, the device comprises in addition a third proximity sensor designed to supply a signal representative of the position or the variation of position of an area of the wire with respect to a second flat vertical measuring surface securedly affixed to the lower reference system and perpendicular to the first flat vertical measuring surface, the device then enables the variations of three-dimensional co-ordinates of the lower reference system with respect to the upper reference system to be accessed. It enables the verticality between the two reference systems to be checked.
A metrological wire is a wire with a low thermal expansion coefficient in the measurement field. Ideally this wire must also have a high longitudinal stiffness so as to only undergo a very small elongation when it supports the pendulous weight. Invar constitutes an alloy combining the mechanical properties sought for. Alternatively, the wire can be made of piano wire or any other material with a mechanical strength, geometry and resistivity equivalent to invar. Another alternative is a carbon fibre wire. However the elongation of the wire, which is greater than for invar, and its variations according to the temperature and humidity, then have to be taken into consideration. In practice, the latter solution will be able to be implemented in dry environments with a stable temperature. The pendulous weight must also be chosen to suit the wire used so as to only generate a very slight elongation while keeping the wire taut.
Preferably at least one of the proximity sensors is a capacitive sensor, sensors of this type having proved their qualities of precision, ease of implementation and reliability over long periods, so that they prove particularly well suited for the type of applications envisaged in surveillance of civil engineering structures. Various types of capacitive measurements can be implemented. If the metrological wire used is electrically conducting, a short current impulse can for example be sent on the wire with a time constant of about one nanosecond. This impulse induces electrostatic charges between two metal plates by influence. Measurement of the charge difference provides quantitative information on the deviation of the wire with respect to the mid-plane between the two plates. If the wire is constituted by a material with a high electrical resistivity, for example carbon fibres, the second and third proximity sensors are then advantageously formed by distance meters of the type described in greater detail in the document FR 2,741,147. Naturally, other proximity measuring principles can be adopted by the metrology specialist.
Preferably, the device further comprises means for processing the signals supplied by the proximity sensor or sensors, comprising means for sampling at a given frequency and computing means enabling an average of the signal taken over a given number of samples to be calculated. The sampling frequency is chosen so as to dampen out the variations due to the residual pendulum phenomenon and therefore is chosen at a value greater than twice the frequency proper of the pendulum. Should the variations due to the longitudinal vibrations of the pendulous weight induced by the stiffness of the wire also require to be dampened out, it may be useful to select a frequency twice the longitudinal frequency proper of the spring weight system formed by the pendulous weight and the wire. However, this variation proves in practice to be insignificant provided that care has been taken to choose a wire of sufficient stiffness. The device enables continuous monitoring by calculation of a moving average. Alternatively, the processing means comprise a software storing the successive low points and high points of oscillation of the pendulum in memory and then computing an average from these points over a time interval greater than the pendulum period. The value obtained is retained if the standard deviation of the set of samples is lower than a preset threshold value above which it is considered that the pendulum is disturbed.
According to one embodiment, the upper reference system and/or lower reference system comprise a metrological sphere centred on a chamfered metrological bore and/or a measuring pot designed to be integrated in an altimetric measuring network by hydrostatic means and/or an electronic GPS reference system and/or an altimetric reference system with a taut horizontal wire.
The different levelling reference systems envisaged are not necessarily redundant in so far as they are each designed to enable levellings with different means and connections to different networks with different precisions. In practice, it is in fact rare for a structure to be equipped with a totally unified measurement system. It is therefore useful to leave some scope in the choice of levelling method.
According to one embodiment, the upper reference system and/or lower reference system are securedly affixed to a connection unit for connection to a taut-wire axial alignment measurement network. The document FR 2,741,147 describes an example of a device of this type comprising a resistive wire stretched along a longitudinal reference axis, arranged near to elements to be aligned and dynamically earthed. A measuring electrode equipped with a guard electrode and arranged in the measurement plane situated close to the taut wire is excited by an AC signal. A capacitive measuring bridge delivers a measurement signal representative of the position of the electrode with respect to the wire in a direction perpendicular to the measurement plane.
According to one embodiment, the upper reference system and/or lower reference system are securedly affixed to a connection unit to an absolute or relative distance measurement network. The connection unit can then be formed by a support bore of a metrological sphere. The absolute measurement can be obtained by measurement with the xe2x80x9cdistinvarxe2x80x9d type distance meter developed by the CERN. Alternatively, absolute or relative distance measurements can be obtained by means of an interferometer.
The device therefore presents a great modularity.